Downhole force and torque sensing system and method

ABSTRACT

A system and method for sensing at least one force on a downhole tool connected in a workstring, according to which a mandrel is connected in the workstring and is subjected to the force. Two or more sensors sense axial or torsional force on the mandrel and are connected in an electrical circuit to convert the sensed force to an output signal.

BACKGROUND

[0001] This invention relates to a system and method for sensing and storing force and torque data on a downhole tool in a downhole oil and gas recovery system.

[0002] Many tools are inserted downhole in a wellbore in an oil and gas recovery system to perform various functions in the recovery process. After many of these tools have been inserted downhole, they require the application of weight and/or torque to operate. For example, in order to “set” a typical downhole retrievable packer, the pipe, or workstring connected to the packer, must be picked up, rotated, and then set back down. After it is set in this manner, the packer is subjected to various other forces such as hydraulic forces, as well as forces caused by thermal expansion and contraction that occur during a cementing or stimulation treatment. Since these forces may change the setting force on the packer and may otherwise adversely affect its operation, it is important that the forces be sensed and their values either stored or transmitted to the surface in real time so as to permit remedial action.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is an elevational view of a downhole tool including an embodiment of a system according to the invention.

[0004]FIG. 2 is a partial schematic, enlarged, side view of a mandrel of the downhole tool of FIG. 1.

[0005]FIG. 3 is a cross-sectional view taken along the line 3-3 of FIG. 2.

[0006]FIG. 4 is a partial schematic, enlarged, view of the opposite side of the mandrel of FIG. 2.

[0007] FIGS. 5-7 are views similar to those of FIGS. 2-4, respectively, but depicting the mandrel rotated ninety degrees from the positions of FIGS. 2-4, respectively.

[0008]FIGS. 8 and 9 are electrical circuits employed in the above system.

DETAILED DESCRIPTION

[0009] Referring to FIG. 1, a tubular mandrel is shown in general by the reference number 10 and forms part of a workstring that is inserted downhole in a wellbore, or the like. A retrievable packer 12, or other downhole tool, is located immediately below the mandrel 10 with the corresponding ends of the mandrel 10 and the packer 12 being connected in any conventional manner. It will be assumed that, when inserted downhole and set, the packer 12, and therefore the mandrel 10, will be subjected to the forces discussed above. The mandrel 10 is thus a load-bearing member of the packer 12 subject to the forces experienced by the packer 12 when it is connected in the workstring. It is understood that other downhole tools (not shown) can also be connected in the workstring.

[0010] A series of batteries 14 are angularly spaced in openings formed inside mandrel 10 and are attached to the mandrel 10 in any conventional manner. A printed circuit board 16 is mounted to the outer surface of the mandrel 10 in any conventional manner and is connected to the batteries 14 for receiving electrical power. An outer tubular case 18 extends over the mandrel 10 and the circuit board 16. It is understood that one or more seal rings can be provided between the case 18 and the mandrel 10.

[0011] A plurality of strain sensors are located on the outer surface of the mandrel 10 and between the mandrel 10 and the case 18. The sensors are not shown in FIG. 1 due to scale limitations, but are shown in detail in FIGS. 2-4. In particular, a pair of axially-spaced sensors 20 and 22 (FIG. 2) are mounted to an exterior surface area of the mandrel 10, and an additional pair of axially-spaced sensors 24 and 26 (FIG. 4) are mounted to an exterior surface area of the mandrel 10 which is diametrically opposite the first-mentioned surface area. The axes of the sensors 20 and 24 extend parallel to the axis of the mandrel 10 and the axes of the sensors 22 and 26 extend perpendicular to the axis of the mandrel 10.

[0012] As better shown in FIGS. 5 and 6, a pair of axially-spaced sensors 30 and 32 are mounted to an exterior surface area of the mandrel 10 and are angularly displaced approximately ninety degrees from the sensors 20 and 22 and from the sensors 24 and 26. Also, as shown in FIGS. 6 and 7, a pair of axially-spaced sensors 34 and 36 are mounted to an exterior surface area of the mandrel 10 diametrically opposite the sensors 30 and 32, and therefore also approximately ninety degrees from the sensors 20 and 22 and from the sensors 24 and 26. The respective axes of the sensors 30, 32, 34, and 36 extend at an angle to the longitudinal axis of the mandrel 10, which, in the example shown, is approximately forty-five degrees. The sensor 30 extends perpendicular to the sensor 32 and the sensor 34 extends perpendicular to the sensor 36.

[0013] Each sensor 20, 22, 24, 26, 30, 32, 34, and 36 can be in the form of a metal foil strain gauge whose resistance varies in response to various forces applied thereto, in a conventional manner.

[0014] The disposition of the axes of the sensors 20 and 24 parallel to the axis of the mandrel 10, and the disposition of the axes of the sensors 22 and 26 perpendicular to the axis of the mandrel 10 enables the sensors 20, 22, 24, and 26 to respond to axial compression and tension along the mandrel 10. Also, the angular disposition of the sensors 30, 32, 34, and 36 enable them to respond to torsional forces on the mandrel 10.

[0015] The sensors 20, 22, 24, and 26 are connected in an electrical circuit, shown in general by the reference numeral 40 in FIG. 8, which is configured in a conventional Wheatstone bridge configuration. The respective outputs of the sensors 20, 22, 24, and 26 of the circuit of FIG. 8, are related to the applied tensile and compression loads on the mandrel 10 according to the following: $\frac{V_{o}}{V} \approx \frac{{{FP}\left( {1 + v} \right)} \times 10^{3}}{2{EA}}$

[0016] whereby:

[0017] V_(o) is the output voltage from the bridge 40

[0018] V is the excitation voltage to the bridge 40

[0019] v is Poisson's ratio

[0020] P is the applied load

[0021] F is a gauge factor for the strain gauge (usually =2)

[0022] E is Young's modulus of the mandrel 10 material

[0023] and

[0024] A is the cross sectional area of the mandrel 10.

[0025] When the circuit 40 is provided with excitation voltage to the sensors 20, 22, 24, and 26, the measured output voltage is representative of the applied tension and compression to the mandrel 10. In the event the mandrel 10 is subject to bending or torsional forces, the strains due to bending and torsion applied to the sensors 20 and 24 and to the sensors 22 and 26 cancel, thus rendering the circuit 40 insensitive to these forces. Also, the circuit 40 is insensitive to any changes in temperature since any temperature dependent changes in the resistance of the sensors 20, 22, 24, and 26 are cancelled.

[0026] The sensors 30, 32, 34, and 36 are connected in an electrical circuit, shown in general by the reference numeral 42 in FIG. 6 which is also configured in a conventional Whetstone bridge configuration. The respective outputs of the sensors 30, 32, 34, and 36 of the circuit of FIG. 9, are related to the applied torsional loads on the mandrel 10 according to the following: $\frac{V}{V_{o}} = {\frac{2{FTR}}{\pi \quad {E\left( {R^{4} - r^{4}} \right)}}\left( {1 + v} \right)}$

[0027] whereby, in addition to the variables defined above:

[0028] T is the applied torque

[0029] R is the outside radius of the mandrel 10

[0030] r is the inside radius of the mandrel 10.

[0031] When the circuit 42 is provided with excitation voltage to the sensors 30, 32, 34, and 36, measurement of the output voltage is representative of the applied torsion to the mandrel 10. Due to the angular disposition of the axes of the sensors 30, 32, 34, and 36 relative to the axes of the mandrel 10, and the design of the circuit 42, the circuit 42 is insensitive to bending, tensile loads, and compressive loads on the mandrel 10. Also, the circuit 42 is insensitive to any changes in temperature since any changes to the sensors 30, 32, 34, and 36 corresponding to axial load, bending, or temperature effects are cancelled.

[0032] It is understood that the circuit board 16 can include hardware and software to provide excitation voltage to circuits 40 and 42, convert the measured output voltages to digital form, and store the measurements at predetermined intervals into nonvolatile memory. In addition the circuit board 16 can include recording devices that record the compression, tension, and torsion applied to the mandrel 10 as a function of time; as well as a telemetry system to transmit the measured output voltages corresponding to the measured values of forces on the mandrel 10 to the surface in real time for further processing.

[0033] It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, the particular type and relative orientation of the sensors can be varied within the scope of the invention. Also, one or more sensors can be utilized for the purpose of sensing only tension, only compression, or only torque on the mandrel 10, or any combination thereof. Further, the mandrel 10 can be located in a different location in the workstring relative to the packer 12 than described above, and can be located relative to other tools in the workstring so that the forces on the latter tools can be measured. Moreover, the angle that the axes of the sensors extend to the longitudinal axis of the mandrel 10 can be varied, and the above equations would be varied accordingly.

[0034] Although only one exemplary embodiment of this invention has been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. 

What is claimed is:
 1. A system for sensing force on a downhole tool connected in a workstring, the system comprising: a load-bearing member of the downhole tool; a plurality of sensors mounted on the load-bearing member and oriented in a manner to sense a force acting on the load-bearing member; and an electrical circuit connected to the sensors to convert the sensed force to an output signal corresponding to the force.
 2. The system of claim 1 wherein the load-bearing member is a mandrel.
 3. The system of claim 2 wherein the sensors are oriented and connected in the circuit in a manner so that sensed bending and torsional forces acting on the mandrel are cancelled.
 4. The system of claim 2 wherein the sensors are oriented and connected in the circuit in a manner so that the circuit is insensitive to temperature changes.
 5. The system of claim 2 wherein the sensors comprise: a first pair of sensors mounted on a first portion of the mandrel; and a second pair of sensors mounted on a second portion of the mandrel diametrically opposite the first pair of sensors.
 6. The system of claim 5 wherein the first and second pairs of sensors are oriented in a manner to sense axial force acting on the mandrel.
 7. The system of claim 6 further comprising: a third pair of sensors mounted on a third portion of the mandrel; and a fourth pair of sensors mounted on a fourth portion of the mandrel diametrically opposite the third pair of sensors, wherein the third and fourth pairs of sensors are oriented in a manner to sense torsional force acting on the mandrel.
 8. The system of claim 7 wherein the third and fourth pairs of sensors are angularly spaced from the first and second pairs of sensors.
 9. The system of claim 8 wherein the third and fourth pairs of sensors are angularly spaced approximately ninety degrees from the first and second pairs of sensors.
 10. The system of claim 7 wherein the third and fourth pairs of sensors are oriented and connected in a manner so that sensed bending and axial forces on the mandrel are cancelled.
 11. The system of claim 7 further comprising an additional circuit connected to the third and fourth pairs of sensors to convert the sensed torsional forces to an additional output signal.
 12. The system of claim 11 further comprising a transmission system for transmitting the additional output signal to the surface.
 13. The system of claim 11 further comprising a recording system for recording the additional output signal.
 14. The system of claim 11 wherein the additional circuit is connected in a Wheatstone bridge configuration.
 15. The system of claim 1 wherein the force is a torsional force.
 16. The system of claim 1 wherein the force is an axial force.
 17. The system of claim 16 wherein the axial force is tension and/or compression.
 18. The system of claim 1 wherein the circuit is connected in a Wheatstone bridge configuration.
 19. The system of claim 1 further comprising a transmission system for transmitting the output signal to the surface.
 20. The system of claim 1 further comprising a recording system for recording the output signal.
 21. A method for sensing force on a downhole tool connected in a workstring, the method comprising the steps of: mounting a plurality of sensors on a load-bearing member of the downhole tool; orienting the sensors in a manner to sense a force acting on the load-bearing member; and connecting an electrical circuit to the sensors to convert the sensed force to an output signal corresponding to the force.
 22. The method of claim 21 wherein the load-bearing member is a mandrel.
 23. The method of claim 22 wherein the sensors are oriented and connected in a manner so that sensed bending and torsional forces acting on the mandrel are cancelled.
 24. The method of claim 22 wherein the sensors are oriented and connected in a manner so that the circuit is insensitive to temperature changes.
 25. The method of claim 22 wherein the circuit is connected in a Wheatstone bridge configuration.
 26. The method of claim 22 wherein the step of orienting the sensors comprises the steps of: mounting a first pair of sensors on a first portion of the mandrel; and mounting a second pair of sensors on a second portion of the mandrel diametrically opposite the first pair of sensors.
 27. The method of claim 26 wherein the first and second pairs of sensors are oriented in a manner to sense axial force acting on the mandrel.
 28. The method of claim 27 further comprising the steps of: mounting a third pair of sensors on a third portion of the mandrel; and mounting a fourth pair of sensors on a fourth portion of the mandrel, wherein the third and fourth pairs of sensors are oriented in a manner to sense torsional force acting on the mandrel.
 29. The method of claim 28 wherein the third and fourth pairs of sensors are angularly spaced from the first and second pairs of sensors.
 30. The method of claim 29 wherein the third and fourth pairs of sensors are angularly spaced approximately ninety degrees from the first and second pairs of sensors.
 31. The method of claim 28 wherein the third and fourth pairs of sensors are oriented and connected in a manner so that sensed axial and bending forces on the mandrel are cancelled.
 32. The method of claim 28 further comprising the step of transmitting the output signal to the surface.
 33. The method of claim 32 further comprising the step of utilizing the transmitted output signal to control the operation of the downhole tool.
 34. The method of claim 28 further comprising the step of recording the output signal.
 35. The method of claim 28 further comprising the step of providing an additional circuit connected to the third and fourth pairs of sensors to convert the sensed torsional force to an additional output signal.
 36. The method of claim 35 further comprising the step of transmitting the additional output signal to the surface.
 37. The method of claim 36 further comprising the step of utilizing the transmitted additional output signal to control the operation of the downhole tool.
 38. The method of claim 35 further comprising the step of recording the additional output signal.
 39. The method of claim 35 wherein the additional circuit is connected in a Wheatstone bridge configuration.
 40. The method of claim 21 wherein the force is tension and/or compression. 